This invention relates to the pulping and grading of waste material and more particularly to the pulping and grading of waste stock intended for use in paper or board making machines.
In one known apparatus for pulping and grading such waste material, the stock to be so pulped and graded is introduced through a tangential inlet to a generally cylindrical chamber. An impeller generally disposed in an end wall remote from the tangential inlet provides for pulping and rotational movement of the stock within the chamber. Accepts pass through a screen in the wall behind the impeller. In an apparatus of this general type disclosed in U.S. Pat. No. 3,844,488, heavy rejects are removed from the chamber through an annular recess while light rejects are permitted to accumulate in the central core of the chamber near the wall opposite the impeller. Periodically the light rejects are tapped off through an axial outlet in the wall opposite the impeller. This apparatus is designed under the assumption that such a pulping and grading machine acts in the nature of a vortex separator.
The rotational movement of the stock within the chamber results in large centrifugal forces and a significant increase in pressure near the outer periphery or cylindrical wall. As a result of the inlet line being located in the outer periphery of the chamber, a significantly large pressure head is required to force the stock into the chamber for processing. Further, the fact that the exact rate of accumulation of lightweight rejects adjacent the central portion of the wall opposite the impeller is not known, and can vary throughout the operation of the apparatus, it is basically guesswork how often and how long the lightweight rejects port should be opened.